Ferritin solutions were microinjected into the ground cytoplasm of intact amebae. At several time-intervals after injection the cells were fixed and the distribution of the protein in various organelles was studied with the aid of the electron microscope. Individual molecules of ferritin were found randomly dispersed throughout the ground cytoplasm and the ground nucleoplasm. Within the mitochondria, the ferritin was localized between the outer and inner membranes. Aggregates of ferritin were found in vacuoles, some of which could be identified as food vacuoles. The findings, which provide evidence for a rapid penetration of large molecules into the nucleus, the outer compartment of the mitochondria, and the digestive vacuoles, are discussed in relation to other reports on intracellular permeability.

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